Untempered systems

Figure 3.3 BEHAVIOUR OF TEMPERED AND UNTEMPERED SYSTEMS DURING RELIEF...

Gassy systems are "untempered". Removal of gas from the relief system will not stop the temperature from rising and the volumetric rate of gas generation will continue to increase. The- relief system should be designed to cope with the maximum rate of gas generation that can occur before the vessel empties. For untempered systems, it is important to check (by testing) whether or not, as the temperature rises, secondary reactions or decompositions occur (see also Figure... 

Hybrid systems may be either tempered or untempered. Generally, untempered systems require much larger relief systems than tempered systems. It is often important that advantage is taken of this in the design of relief systems for tempered hybrids. 

Figure 3.3 shows the typical behaviour of both tempered and untempered systems during relief, with a properly designed relief system. The information that is required for the purposes of relief systerrrdesign will depend upon how the system pressure is generated and whether or not the system is tempered. 

For untempered systems, it is generally conservative to assume initial single-phase gas relief followed by two-phase relief at the peak reaction rate. However, for the hand calculation method given in Chapter 7, it is safe to assume two-phase relief. This is also explained in 4.3.2, Subsection (1) ... 

Gassy systems are untempered. This means that the operation of the relief system cannot control the rate of the runaway reaction, but simply acts to remove material from the reactor. For untempered systems, homogeneous flow in the reactor (see... 

For an untempered system, the relief sizing method should normally use the peak reaction rate. Great care must be taken if the relief sizing method makes use of the reaction rate when the relief.system first operates, as this is less than the peak rate. In such cases, there is danger that. conditions later during the relief process.will be worse, and require a larger relief size. 

For untempered systems, it is/worthwhile considering the use of bottom venting (dumping), rather than relief from the top of the reactor, since this is likely, to require a smaller system. In either case, the safe disposal of,the vented material should be considered (see Chapter 11). Relief from the bottom of the reactor may be a poor option if a relief system is also-required at the topjof the reactor for other process reasons. Operation of the relief system at the top of the vessel would reduce the pressure available to remove ihe contents of the reactor via the bottom relief system. This is discussed further below. t f. . ... 

It is important to realise that, for an untempered system, the reaction rate depends primarily on the temperature and NOT on the pressure. There is no simple relationship between pressure and temperature for an untempered system during relief. f . ... 

Where it is uncertain whether thle system is inherently foamy, it is recommended that the worst case assumption is used (see 4.3.2(1)). For tempered systems, the worst case will be inherent foaminessl Where tempered systems are not inherently foamy, the level swell calculations described in this Annex may lead to a reduction in calculated relief system size. For untempered systems, the worst case is vapour/ liquid disengagement causing reduced mass loss from the reactor during relief. In this case, dynamic simulation (see A3.4) may be needed to take account of level swell in relief sizing. 

The method assumes that the gas is ideal and that homogeneous two-phase relief occurs once the relief system operates. This assumption is potentially non-conservative for untempered systems and, the method should only be used where it is known that homogeneous vessel flow occurs (e.g. for inherently foamy systems)1111. It should not be used if there is external heat input to the reactor, or if the rate of any continuing feed streams is significant. 

This method1161 is the recommended D1ERS method for untempered systems, and is given in detail for that case in 7.3. The method can also be safely used for tempered systems but tends to greatly oversize unless the available overpressure during relief is very small. At zero overpressure, Leung s method for vapour pressure systems (see equation (6.5)) is identical with this method. 

For a tempered system, this rate can be evaluated at the relief pressure because the relief system will then be designed to limit the pressure (and temperature and reaction rate) to this value. For untempered systems, the peak rate attained during the course of the runaway must be used. 

For untempered systems, pressure relief cannot control the temperature rise caused by the runaway. The reacting mixture will therefore increase in temperature and reaction rate during relief. The steady-state relief sizing calculation needs to be performed at the peak reactipn rate, which is normally close to the end of the reaction. The minimum relief ji system size will be obtained if the calculation is performed at the maximum accumulated pressure. 

The required relief rate for untempered systems is given by ... 

For untempered systems, the DIERS sizing equation (see 7.3) can be conservative, unless relief is of gas alone until close to the peak reaction rate. However, it is important to correctly account for any dissolved gas in the experimental test151. Methods which account for early loss of reactants from the reactor, by two-phase relief, should be regarded as best estimate methods rather than conservative calculations. 

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